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Astronomy team probes edge of supermassive black hole

May 28, 2009

Astronomy team probes edge of supermassive black hole

University Park, Pa. — A supermassive black hole lurking deep in the heart of a distant active galaxy has been probed more closely than ever before by a team of astronomers that includes Penn State Professor of Astronomy Niel Brandt. Using new X-ray data from the European Space Agency's XMM-Newton satellite, the team observed the galaxy — known as 1H0707-495 — for four 48-hour-long periods, revealing the innermost depths of the galaxy.

"We now can start to map out the region immediately around the black hole," said Andrew Fabian of the University of Cambridge, who headed the observations and analysis. A research paper describing the team's discoveries will be published on May 28 in the journal Nature.

"The black hole at the heart of this galaxy appears to be eating the material within its reach at a remarkably high rate," Brandt said. "Our observations reveal that the black hole appears to be spinning very rapidly and is eating matter so quickly that it verges on the theoretical limit of its eating ability, swallowing the equivalent of two Earths per hour."

This galaxy has puzzled X-ray astronomers for more than a decade because of the extreme properties of its X-ray spectrum and the strong and rapid variability of its energy output. "Our new long-look observations with XMM-Newton have cracked one this galaxy's main puzzles, the basic nature of its X-ray spectrum," Brandt said. "Our analyses of these new observations point the way forward to resolving this galaxy's remaining mysteries." Brandt contributed to the interpretation of the X-ray analysis results from the team's intense observations of the galaxy.

As matter swirls into a supermassive black hole, X-rays are produced that light up and reflect off of the matter before it disappears into the black hole. Iron atoms in this flow of matter imprint characteristic iron lines on the reflected light, and the extreme forces distort these lines in a number of characteristic ways. The lines are affected by the speed of the orbiting iron atoms, the energy required for the X-rays to escape the black hole's gravitational field, and the spin of the black hole itself. All these features in the new XMM-Newton observations show that the astronomers are tracking matter to within twice the radius of the black hole itself.

XMM-Newton detected two super-bright features of the reflected iron emission that previously had not been seen together in an active galaxy. Known as the iron L and K lines, these features can be super-bright only if there is a high abundance of iron. Finding both the iron L and K lines in this galaxy suggests that the core is much richer in iron than the rest of the galaxy. The team's painstaking statistical analysis of the galaxy's direct X-ray emission, which varies in brightness with time, revealed a time lag of 30 seconds between changes in the X-ray light observed directly, and the X-ray light seen in its reflection from the disk. This delay in the echo enabled the astronomers to measure the size of the reflecting region, which leads to an estimate of the mass of the black hole at about 3 to 5 million solar masses.

Brandt and Fabian are long-time collaborators on studies of the remarkable X-ray spectral and variability properties of galaxies such as the one in this study, which are known as Narrow-Line Seyfert 1 galaxies. Using their new technique, the astronomers are continuing to track the galaxy's black hole, which they describe as a messy eater. "Accretion is a very messy process because of the magnetic fields that are involved," Fabian said.

"The new technique that our team developed can take us to previously unseen regions at a black hole's very edge," Brandt said. "This technique will enable us to map out the gloriously complex feeding frenzy of this and other supermassive black holes throughout the universe."